TI - HIPK2 or p53 knockdown recover endothelial cell survival . AB - To functionally investigate this aspect endothelial cells with a reduced content of HIPK2 or p53 were generated by transient or stable RNA interference . Figure 4a , left and right panels , shows the efficacy of HIPK2 knock-down ( HIPK2i ) , obtained by specific short interfering RNA oligos , at protein ( left ) and RNA level ( right ) . Figure 4b , left panel shows that in this condition Ser46 phosphorylaTION on p53 was abolished in spite of the presence of Bleo and/or SS compared to control cells (CTRi) in which p53 Ser46 PHOSphorylation increased about 9 fold above basal level ( Figure 4b , right panel ) . Remarkably , in cells with a reduced content of HIPK2 , the effect of SS on p21waf1 , cip1 , sdi1 expression was restored in the presence of Bleo as indicated by western blotting ( Figure 4b , left ) and densitometric analyses ( right ) . To provide insights about the functional role of HIPK2 and that of p53 in the Bleo and SS -dependent cyto-toxicity a series of experiments were performed in cells in which RNA interference was obtained by retroviruses bearing short-hairpin interfering oligonucleotides directed to HIPK2 ( sh-HPK2 ) or p53 ( sh-p53 ) . Figure 4c , shows that endothelial cells with a reduced HIPK2 content were significantly resistant , compared to their pSuper control , to the induction of the cell death triggered by laminar SS in the presence of Bleo . As expected , similar results were obtained in cells with a reduced content of p53 ( figure 4d and e ) . These results indicate that the coincident treatment with SS and Bleo realizes a condition in which HIPK2 is activated an negatively influences the SS -dependent effect on p21waf1 , cip1 , sdi1 which is important for the pro-survival response to laminar flow .